The invention relates to ceramic filters and in particular to a specific design of a ceramic filter for use with a radio-beacon receiver. Radio-Beacon receivers are commonly used in certain Differential Global Positioning Systems (DGPS) for accurate navigational and position-locating needs. The international DGPS radio-beacon networks operate in the rf-band of 283-kHz to 325-kHz. To produce a quality radio-beacon receiver, the front-end filter is required to exhibit high-Q properties to provide high selectivity. Conventional beacon receivers based on prior art design techniques have relied upon multiple-pole discrete component ladder-type front-end filter networks, but these had difficulty in maintaining high attenuation in the stop-band whilst also achieving a narrow pass-band with low ripple and low insertion-loss. Such conventional filter networks also imposed the use of very tight tolerance discrete components and quite often invoked the need for custom manufactured components, particularly for the inductor elements. With these impositions and the constraints of critical layout design to achieve desired characteristics, the conventional solutions proved both costly and somewhat unpredictable.
FIG. 1 is a block diagram of a prior art system for implementing a radio-beacon DGPS receiver. This design entails a combination of analogue processing and digital processing stages. A front-end band pass filter 10 is used to select signals having a frequency in the range of 283 kHz to 325 kHz. The filtered signal is then provided to a multiplier 11, an Intermediate Frequency (IF) filter 12 and an amplifier 14 to perform dual down-conversion. Exemplary dual down-conversion converts the input signal from 283-kHz to 325-kHz down to 43-kHz to 85-kHz. The down-converted signal is then applied as input to an A-D converter 16 and a digital signal processor 18. Feedback to the amplifier 14 is provided from the digital signal processor 18 through D-A converter 20.
An exemplary embodiment of the invention is a novel ceramic filter for use as a front-end filter in an innovative radio-beacon DGPS receiver. The filter includes a plurality of piezo-electric resonant stacks in a multi-stage topology to emulate a discrete component ladder network, representing a band-pass filter derived from a combination of a modified Butterworth filter and Elliptical filter, having a pass-band center frequency at about 304 kHzxc2x11.5 kHz. The ceramic filter uses square and circular shaped ceramic piezo-electric resonant elements to provide enhanced performance and reduced size. The square and circular piezo-electric resonant elements, sandwiched between spring-plates, are arranged in horizontal layers. The filter stages are serially linked, at the necessary stage boundaries, with the spring-plates at the respective layers joined by tabs.